The light output of a light emitting diode (LED) is generally controlled by regulating a current level of a LED current through the LED. The LED current may be further modulated with, e.g. a pulse width modulation (PWM) scheme. In such a PWM-scheme, the LED receives the LED current in a periodic sequences of pulses of a certain width, while the width of the pulses is modulated from a first pulse width to a second pulse width when the effective light output is to be changed from a first light output level to a second light output level.
A LED drive method and a LED drive circuit thus generally comprise a current source, providing a constant current or an oscillating current with an average current level, and a switch associated with the LED in order to control a path of the current and in order to achieve the pulse width modulation of the LED current.
The switch may be in series with the LED, thus controlling the path of the current by interrupting the path of the current in order to achieve the pulse width modulation.
The switch may alternatively be in parallel with the LED, which will be referred to as a bypass switch. The bypass switch controls the path of the current by either guiding the path of the current through the LED or guiding the path of the current through a bypass path parallel to the LED in order to achieve the pulse width modulation. One of the advantages of such a bypass switch approach is that the current continues to flow, either through the LED or though the bypass path, which allows the use of very efficient current sources, such as a switch-mode current source. This is especially advantageous when a plurality of LEDs are to be operated at a common current level but with a possibly different pulse width between different LEDs from the plurality of LEDs. The LEDs may then be arranged in a plurality of LED segments connected in series, each LED segment comprising a single LED or two or more LEDs, the two or more LEDs preferably arranged in series, and each of the LED segments being associated with a bypass switch in parallel to the corresponding LED segment. By operating the bypass switches independently, the effective light output of each of the LED segments may be varied independently.
An example of a current source is described in WO2004100614A1. WO2004100614A1 describes a LED current control method and circuit for accurately and quickly regulating the mean amperage of LED current during all operating conditions including a change in the input line of a power source or in a change in a load of the LED network.
The method comprises controlling the LED current to oscillate, e.g. in a triangular or saw-tooth manner, between a peak amperage and a valley amperage, with the mean amperage being the average of the peak amperage and the valley amperage, by an alternate controlling of an increase and a decrease of the LED current in response to each crossover by a converter current sensing voltage of a lower trip voltage and an upper trip voltage in a negative and a positive direction respectively. A circuit using such a method may be referred to as an example of a switch-mode converter with hysteretic control on the LED current. The peak-to-valley range of the peak amperage to the valley amperage may be referred to as the hysteretic current window. The peak-to-valley range of the upper trip voltage to the lower trip voltage may be referred to as the hysteretic voltage window, or, in short, the hysteretic window.
The method and circuit thus achieve regulating the mean current level independent of the operating conditions. In particular, when the method and circuit are used to operate a LED circuit arrangement comprising a plurality of LED segments with corresponding bypass switches in an arrangement as described above. Operating the bypass switches to vary the light output of the individual LED segments results in a variation of the load of the LED circuit arrangement. The switch-mode converter with hysteretic control is well suited to accurately and quickly deliver a current with a substantially constant mean current level to such a LED circuit arrangement with varying load due to the operation of the bypass switches.
However, using the method of WO2004100614A1 results in a varying frequency of the oscillation of the LED current when the load of the LED circuit arrangement is varying, e.g. due to the operation of the bypass switches as described above. When the load of the LED circuit arrangement is varying significantly, the frequency variation may be large.
This large variation of frequency has several negative side-effects. For example, the components in an input or output filter of the hysteretic switch-mode converter need to be dimensioned such as to reduce the side-effects to a sufficiently low level for all possible frequencies. The requirements stemming from these side-effects are for example preventing audible noise, preventing visible and potentially annoying fluctuations in the light output of the LEDs, complying to conducted and radiated electromagnetic interference (EMI) regulations, guaranteeing lifetime of electrolytic capacitors, and optimizing core versus conduction losses in inductors. For example, in order to guarantee that the frequency does not, during all operating conditions, move into the range of audible frequencies, which may be annoying, a small inductor is needed in the output filter, which negatively impacts the accuracy of the LED current level. As another example, in order to achieve a small ripple of the LED current when the load of the LED circuit arrangement is varying significantly while being tolerant to changes in the input line of the power source, a large capacitor may be needed to filter the input voltage.